Semiconductor device with surface insulating film

ABSTRACT

A semiconductor device of the present invention includes a semiconductor layer of a first conductivity type having a cell portion and an outer peripheral portion disposed around the cell portion, and a surface insulating film disposed in a manner extending across the cell portion and the outer peripheral portion, and in the cell portion, formed to be thinner than a part in the outer peripheral portion.

TECHNICAL FIELD

The present invention relates to a semiconductor device.

BACKGROUND ART

Semiconductor power devices have conventionally become the focus ofattention, which are mainly used for systems in various powerelectronics fields such as motor control systems and power conversionsystems.

As semiconductor power devices of this type, SiC semiconductor deviceshaving a trench-gate structure have been proposed, for example.

For example, Patent Literature 1 discloses a field effect transistorincluding an n⁺-type SiC substrate, an n⁻-type epitaxial layer (driftregion) formed on the SiC substrate, a p-type body region formed at asurface side of the epitaxial layer, an n⁺-type source region formed ata surface side within the body region, a grid-shaped gate trench formedin a manner penetrating through the source region and the body region toreach the drift region, a gate insulating film formed on the innersurface of the gate trench, a gate electrode embedded in the gatetrench, a source trench formed in a manner penetrating through thesource region and the body region to reach the drift region at aposition surrounded by the grid-shaped gate trench, and a sourceelectrode formed in a manner entering the source trench

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Publication No.2011-134910

SUMMARY OF INVENTION Technical Problem

It is an object of the present invention to provide a semiconductordevice capable of improving the flatness of a surface metal layerwithout sacrificing conventional breakdown characteristics.

Solution to Problem

A semiconductor device of the present invention includes a semiconductorlayer of a first conductivity type having a cell portion and an outerperipheral portion disposed around the cell portion, and a surfaceinsulating film disposed in a manner extending across the cell portionand the outer peripheral portion, and in the cell portion, formed to bethinner than a part in the outer peripheral portion.

According to this arrangement, selectively thinning the surfaceinsulating film of the cell portion allows reducing a difference inlevel (unevenness) between a surface of the surface insulating film anda surface of the cell portion (device surface) when, for example, anopening (contact holes etc.) is formed in the surface insulating film.When metal is buried in said opening to form a surface metal layer onthe surface insulating film, the flatness of the surface metal layer canthereby be improved. Thus, when, for example, a wire is bonded to thesurface metal layer, adhesion between the surface metal layer and thewire can be improved. As a result, the wire can be satisfactorilybonded, so that the wire bonding portion can be improved in reliability.Further, because the surface metal layer is excellent in flatness,destruction of the device by ultrasonic vibration and pressure can beprevented at the time of wire bonding, and a decline in assembling yieldcan be prevented.

On the other hand, the thickness of the surface insulating film of theouter peripheral portion can be designed separately from the thicknessof the surface insulating film of the cell portion. Thus, designing withsuch a thickness so as not to influence the electric field distributionin the outer peripheral portion allows maintaining breakdowncharacteristics. In other words, according to this arrangement, at thetime of an improvement in flatness of the surface metal layer, variationin breakdown characteristics and a dielectric breakdown failure due tothe variation can be prevented.

The semiconductor device may include a gate trench formed at a surfaceside of the cell portion, and a gate electrode buried in the gate trenchvia a gate insulating film, forming a channel at a portion lateral tothe gate trench at ON-time, the outer peripheral portion may have asemiconductor surface disposed at a depth position equal to or deeperthan a depth of the gate trench, and the semiconductor device mayfurther include a voltage resistant structure having a semiconductorregion of a second conductivity type formed in the semiconductor surfaceof the outer peripheral portion.

According to this arrangement, the voltage resistant structure can beformed at a depth position equivalent to or deeper than the depth of thegate trench. The thickness of the semiconductor layer from a bottomportion of the gate trench to a back surface of the semiconductor layercan thereby be made thicker than the thickness from the semiconductorregion that constitutes a voltage resistant structure to said backsurface. As a result, an electric field imposed on a section between thesurface side and back surface side of the semiconductor layer can bemade to be stably shared by the voltage resistant structure. Because astable electric field distribution can accordingly be formed in thesemiconductor layer without depending on the depth of the gate trench,an electric field concentration to the bottom portion of the gate trenchcan be satisfactorily relaxed.

The semiconductor device may further include a gate finger to makecontact with the gate electrode, and the gate trench may include aline-shaped trench that runs across the gate finger under the gatefinger.

According to this arrangement, because trench corner portions (forexample, corners etc., in crossing portions of a grid-shaped trench)where an electric field is likely to concentrate when a gate voltage isapplied are not disposed under the gate finger, the reliability andresistance of the gate insulating film can be improved.

The gate trench may include an inner trench at a portion lateral towhich the channel is formed at ON-time, and an outer trench constitutedof an extension portion of the inner trench, disposed outside withrespect to the inner trench, and the semiconductor device may furtherinclude a second conductivity type-layer formed at side and bottomportions of the outer trench.

According to this arrangement, a depletion layer can be generated, by asecond conductivity-type layer different in conductivity type from thesemiconductor layer, from a junction (p-n junction) between said secondconductivity-type layer and the semiconductor layer. Moreover, becausethe depletion layer keeps equipotential surfaces away from the outertrench, electric fields to be imposed on the bottom portion of the outertrench can be further relaxed. Breakdown in the bottom portion of theouter trench can accordingly be prevented.

The semiconductor device may further include a gate finger to makecontact with the gate electrode, the gate trench may be selectivelyformed in a region under the gate finger, and the semiconductor devicemay further include a high-concentration layer of a first conductivitytype formed in a semiconductor surface of the semiconductor layer wherethe gate trench is not formed in the region under the gate finger,containing an impurity at a higher concentration than that of thesemiconductor layer.

According to this arrangement, the oxidation rate of thehigh-concentration layer having a high impurity concentration can bemade fast as compared with that of the semiconductor layer having alower concentration than that high impurity concentration. Thus, whenthe gate insulating film is formed by thermal oxidization, a thick oxidefilm can be selectively formed in an upper portion of the gate trench ina region under the gate finger. An electric field to be imposed on anupper edge of the gate trench when a gate voltage is applied can therebybe weakened to prevent a dielectric breakdown of the gate insulatingfilm.

The cell portion may include a source region of a first conductivitytype disposed in a manner exposed on a surface of the semiconductorlayer, a channel region of a second conductivity type which is disposedin a manner contacting the source region and in which the channel isformed at ON-time, a drain region of a first conductivity type disposedin a manner contacting the channel region, a second trench selectivelyformed in a source portion defined in a manner including the sourceregion in the surface of the semiconductor layer, and a channel contactregion of a second conductivity type selectively disposed at a bottomportion of the second trench, electrically connected with the channelregion.

The cell portion may include a source region of a first conductivitytype disposed in a manner exposed on a surface of the semiconductorlayer, a channel region of a second conductivity type which is disposedin a manner contacting the source region and in which the channel isformed at ON-time, a drain region of a first conductivity type disposedin a manner contacting the channel region, a second trench selectivelyformed in a source portion defined in a manner including the sourceregion in the surface of the semiconductor layer, a trench buriedportion buried in the second trench, and a channel contact region of asecond conductivity type selectively disposed at a position higher thanthat of a bottom portion of the second trench in the source region,electrically connected with the channel region.

According to this arrangement, a concentration of equipotential surfacesin a vicinity of the bottom portion of the gate trench can be preventedby the second trench, so that a potential gradient, for example, in thevicinity of the bottom portion can be made gradual. Therefore, anelectric field concentration to the bottom portion of the gate trenchcan be relaxed.

Also, because the trench buried portion is buried in the second trench,a difference in level (unevenness) between the source portion and otherparts can be reduced on the surface of the semiconductor layer (devicesurface). When a surface metal layer is formed on said device surface,the flatness of the surface metal layer can thereby be improved. Thus,when, for example, a wire is bonded to the surface metal layer, adhesionbetween the surface metal layer and the wire can be improved. As aresult, the wire can be satisfactorily bonded, so that the wire bondingportion can be improved in reliability. Further, because the surfacemetal layer is excellent in flatness, destruction of the device byultrasonic vibration and pressure can be prevented at the time of wirebonding, and a decline in assembling yield can be prevented.

Further, because the channel contact region is disposed at a positionhigher than that of the bottom portion of the second trench, even whenthere is formed a second trench, contact with the channel region can bereliably made via the channel contact region. In other words, at thetime of an improvement in flatness of the surface metal layer, adegradation in device performance such as gate withstand voltage andcontact performance with the channel region can be prevented.

The trench buried portion may consist of an insulating film formed on aninner surface of the second trench and a polysilicon layer buried insideof the insulating film.

According to this arrangement, the polysilicon layer buried in thesecond trench can be used as an etching stopper, in the case where, forexample, there is formed a surface insulating film made of SiO₂ on thesurface of the semiconductor layer, when selectively etching the surfaceinsulating film to expose the source portion from a contact hole.Therefore, control of the step of said contact etching can besimplified.

The insulating film may be made of any of SiO₂, AlON, Al₂O₃, SiO₂/AlON,SiO₂/AlON/SiO₂, SiO₂/SiN, and SiO₂/SiN/SiO₂.

According to this arrangement, by, for example, forming the gateinsulating film in the same step as that for the insulating film in thesecond trench, a gate insulating film constituted of a materialexemplified in the above can be provided. In this case, providing a gateinsulating film constituted of a high-dielectric-constant (high-k) filmof AlON, Al₂O₃, or the like allows an improvement in gate withstandvoltage, so that device reliability can be improved.

The insulating film may have a SiO₂ film containing nitrogen (N).

According to this arrangement, by, for example, forming the gateinsulating film in the same step as that for the insulating film in thesecond trench, a gate insulating film constituted of a material having aSiO₂ film containing nitrogen (N) can be provided. This gate insulatingfilm can improve channel mobility.

The insulating film may be, at the bottom portion of the second trench,formed to be thicker than a part at a side portion of the second trench.

According to this arrangement, by, for example, forming the gateinsulating film in the same step as that for the insulating film in thesecond trench, the gate insulting film can also be made, for example, atthe bottom portion of the gate trench, thicker than a part at a sideportion of the gate trench. Withstand voltage in the bottom portion ofthe gate trench can thereby be improved.

The polysilicon layer may be made of n⁺-type polysilicon.

According to this arrangement, by, for example, forming the gateelectrode in the same step as that for the polysilicon layer in thesecond trench, a gate electrode constituted of n⁺-type polysilicon canbe provided. The n⁺-type polysilicon has a relatively low sheetresistance, which therefore allows increasing transistor switchingspeed.

The trench buried portion may consist of an insulating layer that fillsback the second trench.

According to this arrangement, because the inside of the second trenchis filled with the insulating layer, a leakage current that flows viathe second trench can be prevented or reduced.

The insulating layer may be made of SiO₂. In this case, the insulatinglayer may be made of SiO₂ containing phosphorus (P) or boron (B).

According to this arrangement, because the melting point of SiO₂ fallsas a result of containing phosphorous or boron, the process for buryingthe insulating film can be simplified. As such SiO₂, for example, PSG(phosphorus silicate glass) or PBSG (phosphorus boron silicate glass)can be used.

The trench buried portion may consist of a polysilicon layer that fillsback the second trench.

According to this arrangement, the polysilicon layer buried in thesecond trench can be used as an etching stopper, in the case where, forexample, there is formed a surface insulating film made of SiO₂ on thesurface of the semiconductor layer, when selectively etching the surfaceinsulating film to expose the source portion from a contact hole.Therefore, control of the step of said contact etching can besimplified.

The polysilicon layer may be made of p⁺-type polysilicon.

According to this arrangement, when, for example, the channel region andthe channel contact region are p-type, these regions can be electricallyconnected by use of a p⁺-type polysilicon layer. Because the length of acurrent path between the channel region and the channel contact regioncan thereby be reduced, a base resistance therebetween can be reduced.As a result, latch-up can be satisfactorily prevented. Further, when thechannel contact region is in contact with the polysilicon layer, acontact resistance therebetween can also be reduced. The reduction incontact resistance also contributes to a reduction in the baseresistance between the channel region and the channel contact region.

The semiconductor device of the present invention may further include asecond conductivity-type layer formed at the bottom portion and a sideportion of the second trench in a manner continuing from the channelregion and the channel contact region.

According to this arrangement, a depletion layer can be generated, by asecond conductivity-type layer different in conductivity type from thesemiconductor layer, from a junction (p-n junction) between said secondconductivity-type layer and the semiconductor layer. Moreover, becausethe depletion layer keeps equipotential surfaces away from the gatetrench, electric fields to be imposed on the bottom portion of the gatetrench can be further relaxed.

The gate electrode may include a base film made of polysilicon formed onan inner surface of the gate trench, and a buried metal containing atleast one of Mo, W, Al, Pt, Ni, and Ti buried inside of the base film.

According to this arrangement, the buried metal can make gate resistancerelatively low, which therefore allows increasing transistor switchingspeed.

The semiconductor device may further include a surface metal layer madeof a metal containing copper (Cu) disposed at a surface side of thesemiconductor layer. In this case, the surface metal layer may containan Al-Cu-based alloy.

According to this arrangement, because the sheet resistance of thesurface metal layer can be reduced, the current density can beincreased.

In the cell portion, a plurality of unit cells may be defined in a gridshape by the gate trench, or may be defined in a striped shape by thegate trench.

The semiconductor layer may be made of SiC, GaN, or diamond.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic plan view of a semiconductor device according to afirst preferred embodiment of the present invention.

FIG. 2 is an enlarged view showing a main part of FIG. 1, in which apart is shown perspectively.

FIG. 3 shows a sectional structure of the semiconductor device in asection taken along of FIG. 2.

FIG. 4 shows a sectional structure of the semiconductor device in asection taken along IV-IV of FIG. 2.

FIG. 5 shows a sectional structure of the semiconductor device in asection taken along V-V of FIG. 2.

FIG. 6 shows a sectional structure of the semiconductor device in asection taken along VI-VI of FIG. 2.

FIG. 7 is a view showing a cell portion in FIG. 3 in an enlarged manner.

FIG. 8 is a view showing a first modification of the cell portion.

FIG. 9 is a view showing a second modification of the cell portion.

FIG. 10 is a view showing a third modification of the cell portion.

FIG. 11 is a view showing a fourth modification of the cell portion.

FIG. 12 is a view showing a fifth modification of the cell portion.

FIG. 13 is a schematic sectional view of a semiconductor deviceaccording to a second preferred embodiment of the present invention.

FIG. 14 is a schematic sectional view of the semiconductor deviceaccording to the second preferred embodiment of the present invention.

FIG. 15 is a schematic sectional view of a semiconductor deviceaccording to a third preferred embodiment of the present invention.

FIG. 16 is a schematic sectional view of the semiconductor deviceaccording to the third preferred embodiment of the present invention.

FIG. 17 is a schematic sectional view of a semiconductor deviceaccording to a first reference embodiment.

FIG. 18 is a schematic sectional view of the semiconductor deviceaccording to the first reference embodiment.

FIG. 19 is a schematic sectional view of a semiconductor deviceaccording to a second reference embodiment.

FIG. 20 is a schematic sectional view of the semiconductor deviceaccording to the second reference embodiment.

FIG. 21 is a schematic sectional view of a semiconductor deviceaccording to a fourth preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS First Preferred Embodiment

Hereinafter, preferred embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 1 is a schematic plan view of a semiconductor device according to afirst preferred embodiment of the present invention.

The semiconductor device 1 includes a SiC-based MISFET (Metal InsulatorSemiconductor Field Effect Transistor). As shown in FIG. 1, thesemiconductor device 1 has, for example, a square chip-shaped contour ina plan view. The chip-shaped semiconductor device 1 is sized to have avertical and horizontal length of about several millimeters in theillustration of FIG. 1. A cell portion 2 and an outer peripheral portion3 disposed around the cell portion 2 are set on the semiconductor device1. The outer peripheral portion 3 is, in the present preferredembodiment, formed in an annular shape in a manner surrounding the cellportion 2, but the outer peripheral portion 3 need not particularly bean annular shape as long as it is a region at an outer side of the chipwith respect to the cell portion 2.

The semiconductor device 1 includes a source pad 4, a gate pad 5, and agate finger 6.

The source pad 4 is disposed in a region over the cell portion 2. In thepresent preferred embodiment, the source pad 4 is formed in, forexample, a square shape in a plan view in a manner coveringsubstantially the whole of the cell portion 2. In a peripheral edgeportion of the source pad 4, a removal region 7 (cross-hatched part inFIG. 1) that surrounds a central region of the source pad 4 is formedalong the outer peripheral portion 3. The removal region 7 isselectively recessed at a part thereof toward the central region of thesource pad 4. The recess is disposed in its entirety in a region overthe cell portion 2, and has the gate pad 5 installed therein.

The gate finger 6 extends, in a position closer to the outer peripheralportion 3 with respect to a boundary between the cell portion 2 and theouter peripheral portion 3, from the gate pad 5 across the entireremoval region 7 along the outer peripheral portion 3. In the presentpreferred embodiment, a pair of gate fingers 6 are formed in a shapesymmetrical with respect to the gate pad 5. Moreover, in the presentpreferred embodiment, the boundary (the same as the boundary L in FIG.2) between the cell portion 2 and the outer peripheral portion 3described above is set, for example, along an inner part with respect tothe gate fingers 6 of the removal region 7.

The cell portion 2 is further formed with a gate trench 8. The gatetrench 8 is, in the present preferred embodiment, selectively formed ina region under the source pad 4 in a manner avoiding a region under thegate pad 5. In this region, the gate trench 8 is formed in a mannerdefining a plurality of unit cells 9. The gate trench 8 may be patternedin, as shown in FIG. 1, a grid shape or in a striped shape. A largenumber of unit cells 9 are thereby regularly arrayed in a matrix shape(in an array) or a striped shape (linearly) in the cell portion 2.Although, this is not shown, the unit cells 9 may be patterned inanother shape such as a honeycomb shape.

Next, an internal structure of the cell portion 2 and the outerperipheral portion 3 of the semiconductor device 1 will be described.

FIG. 2 is an enlarged view showing a main part of FIG. 1, in which apart is shown perspectively. Specifically, a structure in a region underthe source pad 4 and the gate fingers 6 is shown by solid lines, and thesource pad 4 and the gate fingers 6 are shown by broken lines. FIG. 3shows a sectional structure of the semiconductor device in a sectiontaken along III-III of FIG. 2. FIG. 4 shows a sectional structure of thesemiconductor device in a section taken along IV-IV of FIG. 2. FIG. 5shows a sectional structure of the semiconductor device in a sectiontaken along V-V of FIG. 2. FIG. 6 shows a sectional structure of thesemiconductor device in a section taken along VI-VI of FIG. 2.

The semiconductor device 1 includes a substrate (not shown) made ofn⁺-type SiC (for example, having a concentration of 1×10¹⁸ to 1×10²¹cm⁻³) and an n⁻-type epitaxial layer 10 made of n⁻-type SiC (forexample, having a concentration of 1×10¹⁵ to 1×10¹⁷ cm⁻³) formed on thesubstrate. The n⁻-type epitaxial layer 10 is a layer formed by causingSiC to epitaxially grow on a surface of the substrate. In the presentpreferred embodiment, the substrate and the n⁻-type epitaxial layer 10are shown as an example of a semiconductor layer of the presentinvention. Also, the substrate has a thickness of, for example, about250 μm to 350 μm, and the n⁻-type epitaxial layer 10 has a thickness ofabout 3 μm to 20 μm.

The n⁻-type epitaxial layer 10 has a semiconductor surface 11 with adifference in height formed by being selectively dug down at a partthereof. The difference in height of the semiconductor surface 11 is, inthe present preferred embodiment, formed by the gate trench 8 and sourcetrenches 33 (described later) selectively formed in the cell portion 2and the outer peripheral portion 3 and a low step portion 12 selectivelyformed in the outer peripheral portion 3. In the following, thesemiconductor surface 11 where the gate trench 8, the source trenches33, and the low step portion 12 are not formed and which is maintainedat a height position after epitaxial growth is provided as a basesurface 11B, and like a bottom face of the gate trench 8, a bottom faceof the source trench 33, and a bottom face of the low step portion 12,the semiconductor surface 11 formed at a relatively low height positionwith respect to the base surface 11B is provided as a lower surface 11L.

The gate trench 8, in the present preferred embodiment, includes aninner trench 13 that is used as the gate of a MISFET, an outer trench 14disposed on an outer side with respect to the inner trench 13, and acontact trench 15 that is pulled out from the outer trench toward theouter peripheral portion 3 to serve as a contact with a gate electrode16 (described later). The trenches 13 to 15 are integrally formed in amanner communicating with each other.

As shown in FIG. 2, the inner trench 13 is formed in a grid shape by aplurality of line-shaped trenches crossing each other in a mannerdefining a plurality of unit cells 9. Terminal portions of therespective lines of the inner trench 13 are connected to each other bythe outer trench 14. In other words, the outer trench 14 is formed in amanner surrounding the inner trench 13, and extends across between theterminal portions of mutually adjacent lines of the inner trench 13.

The contact trenches 15 are formed in line shapes constituted ofextension portions of the respective lines of the inner trench 13, anddisposed in plural numbers, spaced apart from each other, along theboundary L between the cell portion 2 and the outer peripheral portion3. In addition, the contact trenches 15 need not be provided for everyline of the inner trench 13, and may be provided, for example, for everyother line of the inner trench 13. The line-shaped contact trenches 15are formed in a manner running across the gate fingers 6 in a regionunder the gate fingers 6. In the present preferred embodiment, thecontact trenches 15 have their terminal portions disposed furtheroutside than the gate fingers 6. In other words, the terminal portionsof the contact trenches 15 are sticking out further outside than thegate fingers 6.

Moreover, in the gate trench 8, a gate electrode 16 made of, forexample, polysilicon is buried, and a gate insulating film 17 isinterposed between the gate electrode 16 and the n⁻-type epitaxial layer10.

The gate electrode 16 is, for example, as shown in FIG. 3 and FIG. 4,buried up to the base surface 11B in the inner trench 13 and the outertrench 14 formed at positions separated from a region under the gatefingers 6. The gate electrode 16 is thereby also formed in a grid shape,and an upper face of the respective unit cells 9 is exposed withoutbeing covered with the gate electrode 16. On the other hand, in thecontact trenches 15 formed in a region under the gate fingers 6, thegate electrode 16 has an overlapping portion 18 formed from an openingend of the contact trench 15 in a manner selectively covering the basesurface 11B. The overlapping portion 18 is, in the present preferredembodiment, as shown in FIG. 2, formed along the gate fingers 6 in amanner running across the line-shaped contact trenches 15. As shown inFIG. 5 and FIG. 6, also between the overlapping portion 18 and then⁻-type epitaxial layer 10, a gate insulating film 17 is interposed.

In the cell portion 2, the gate electrode 16 controls formation of aninversion layer (channel) in the unit cell 9. That is, the semiconductordevice 1 has a so-called trench-gate type structured MISFET.

The low step portion 12 is, in the present preferred embodiment, formedacross the entire circumference of the outer peripheral portion 3, andthereby surrounds the cell portion 2. The low step portion 12 is formedwith a depth equal to or deeper than a depth of the gate trench 8. Thus,in the outer peripheral portion 3, the bottom face (lower surface 11L)of the low step portion 12 is formed at a depth position equal to ordeeper than that of the bottom face (lower surface 11L) of the gatetrench 8. That depth is, for example, 0.7 μm to 5 μm with reference tothe base surface 11B, relative to the depth of the gate trench 8 of 0.7μm to 3 μm.

Moreover, in the semiconductor surface 11 having such a difference inheight, n-type and p-type impurity regions are selectively formed.

Specifically, in a surface portion of the n⁻-type epitaxial layer 10, ap-type well 19 (for example, having a concentration of 1×10¹⁶ to 1×10¹⁹cm⁻³) is formed in a manner extending across the cell portion 2 and theouter peripheral portion 3. On the other hand, a region of a portionunder the p-type well 19 in the n⁻-type epitaxial layer 10 is an n⁻-typedrain region 20. In the present preferred embodiment, as shown in FIG.3, the p-type well 19 is continuously formed, from the cell portion 2 tothe low step portion 12 of the outer peripheral portion 3 via a regionunder the gate fingers 6, in such a manner that its bottom portionfollows the base surface 11B. The p-type well 19 is thereby exposed at aside portion of the low step portion 12.

In the p-type well 19, as shown in FIG. 3, FIG. 5, and FIG. 6, ann⁺-type region 21 is formed in a region under the gate fingers 6, andexposed on the base surface 11B of the n⁻-type epitaxial layer 10. Then⁺-type region 21 is a high concentration region (for example, having aconcentration of 1×10¹⁸ to 1×10²¹ cm⁻³) containing an n-type impurity ata higher concentration than that of the n⁻-type epitaxial layer 10. Inthe present preferred embodiment, as shown in FIG. 3, the n⁺-type region21 is continuously formed, from the cell portion 2 to the low stepportion 12 of the outer peripheral portion 3 via a region under the gatefingers 6, in such a manner that its bottom portion follows the basesurface 11B. The n⁺-type region 21 is thereby exposed at a side portionof the low step portion 12.

In the n⁻-type epitaxial layer 10, as shown in FIG. 4 to FIG. 6, ap-type layer 22 (for example, having a concentration of 1×10¹⁶ to 1×10¹⁹cm⁻³) is formed in a region under the gate fingers 6 in a mannercontinuing from the p-type well 19. The p-type layer 22 is, in thepresent preferred embodiment, formed in a manner extending across abottom portion and side portion (including a side portion of theterminal portion) of the contact trench 15, and its inner region is incontact with the contact trench 15 (exposed into the contact trench 15).Also, the p-type layer 22 is, at the bottom portion of the contacttrench 15, formed to be thicker than a part at a side portion of thecontact trench 15.

Also, in the n⁻-type epitaxial layer 10, as shown in FIG. 3 and FIG. 4,a p-type layer 23 (for example, having a concentration of 1×10¹⁶ to1×10¹⁹ cm⁻³) serving as an example of a voltage resistant structure ofthe present invention is formed in the low step portion 12 in a mannercontinuing from the p-type well 19. The p-type layer 23 is, in thepresent preferred embodiment, formed in a manner extending across abottom portion and side portion of the low step portion 12, and itsinner region is in contact with the low step portion 12 (exposed intothe low step portion 12). In a surface portion of the p-type layer 23, ap⁺-type well contact region 24 (for example, having a concentration of1×10¹⁸ to 1×10²¹ cm⁻³) is formed. In the present preferred embodiment,the p⁺-type well contact region 24 is formed in the lower surface 11L ofthe low step portion 12, and formed in an annular shape in a mannersurrounding the cell portion 2.

Also, outside of the p-type layer 23 in the low step portion 12, ap-type guard ring 25 (for example, having a concentration of 1×10¹⁶ to1×10¹⁹ cm⁻³) serving as an example of a voltage resistant structure ofthe present invention is formed. In the present preferred embodiment,the p-type guard rings 25 are formed in plural numbers, spaced apartfrom each other, in a manner surrounding the cell portion 2 in the lowersurface 11L of the low step portion 12.

On a surface of the n⁻-type epitaxial layer 10, a surface insulatingfilm 26 is formed in a manner extending across the cell portion 2 andthe outer peripheral portion 3. The surface insulating film 26 is madeof an insulator such as silicon oxide (SiO₂), for example. The surfaceinsulating film 26 is, in the present preferred embodiment, formed suchthat an inner part 27 on the cell portion 2 becomes thinner than anouter part 28 on the outer peripheral portion 3. In the presentpreferred embodiment, the inner part 27 has a thickness of 5000 Å orless, and the outer part 28 has a thickness of about 5500 Å to 20000 Å.The surface insulating film 26 may be called an interlayer insulatingfilm when a multilayer wiring structure is disposed thereon, which isnot shown in FIG. 2.

In the surface insulating film 26, contact holes 29 to 31 thatselectively expose the respective unit cells 9, the gate electrode 16(overlapping portion 18), and the p⁺-type well contact region 24,respectively, are formed over the entire surface of the n⁻-typeepitaxial layer 10.

On the surface insulating film 26, the source pad 4 and the gate fingers6 are formed.

The source pad 4 is connected collectively to p⁺-type channel contactregions 34 (described later) and n⁺-type source regions 32 (describedlater) of all unit cells 9 and the p⁺-type well contact region 24 viathe respective contact holes 29 and 31. In other words, the source pad 4serves as a common electrode to all unit cells 9. Also, as the materialfor the source pad 4, a metal containing copper (Cu) can be used, andmore preferably, a metal containing an Al—Cu-based alloy is used.Because the sheet resistance of the source pad 4 can thereby be reduced,the current density can be increased. Also, the source pad 4 has athickness (distance from the base surface 11B of the n⁻-type epitaxiallayer 10 to a surface of the source pad 4) of, for example, 4 μm to 5μm. In addition, the source pad 4 may have a contact metal made of, forexample, a laminated structure (Ti/TiN) of titanium (Ti) and titaniumnitride (TiN) at a connection part with the n⁻-type epitaxial layer 10.

The gate fingers 6 are connected to the gate electrode 16 (overlappingportion 18) via the contact hole 30. Also, as the material for the gatefingers 6 and the gate pad 5, similar to that for the source pad 4, ametal containing copper (Cu) can be used, and more preferably, a metalcontaining an Al—Cu-based alloy is used. Using the same material as thatfor the source pad 4 allows simultaneously forming the source pad 4, thegate pad 5, and the gate fingers 6.

Next, the structure of the cell portion 2 will be described in greaterdetail. FIG. 7 is a view showing the cell portion 2 in FIG. 3 in anenlarged manner.

In the cell portion 2, as described above, the plurality of unit cells 9each of which performs a transistor operation are defined in a gridshape by the gate trench 8 (inner trench 13 and outer trench 14). Eachunit cell 9 includes an annular n⁺-type source region 32, an annularsource trench 33 (second trench) surrounded by the n⁺-type source region32, and a p⁺-type channel contact region 34 formed in an island shapeinside the source trench 33. The p⁺-type channel contact region 34 issurrounded by the source trench 33 at its periphery. Also, each unitcell 9 is sized to have a vertical and horizontal length of, forexample, about 3 μm to 10 μm in the illustration of FIG. 7.

Specifically, an n⁺-type source region 32 is formed in a surface portionof the p-type well 19 in the cell portion 2, and exposed on the basesurface 11B of the n⁻-type epitaxial layer 10. In addition, a partwithin the cell portion 2 of the p-type well 19 is a p-type channelregion 35 which is disposed in a manner contacting the n⁺-type sourceregion 32 and in which a channel is formed at the time of a transistoroperation.

Moreover, the gate trench 8 and the source trench 33 are formed in amanner penetrating through the n⁺-type source region 32 and the p-typechannel region 35 (p-type well 19) to reach the n⁻-type drain region 20.The gate trench 8 and the source trench 33 are, in the present preferredembodiment, formed with the same width and the same depth, but may bedifferent in depth from each other. For example, the source trench 33may be shallower or may be deeper than the gate trench 8.

Each unit cell 9 is separated into a prismatic portion 36 surrounded bythe source trench 33 and an annular portion 37 disposed between thesource trench 33 and the gate trench 8 and spaced apart from theprismatic portion 36 by the source trench 33. In the present preferredembodiment, the width W₁ of the annular portion 37 (distance between thesource trench 33 and the gate trench 8) is, for example, 0.5 μm to 2.0μm.

In a top portion of the prismatic portion 36, a p⁺-type channel contactregion 34 (for example, having a concentration of 1×10¹⁸ to 1×10²¹ cm⁻³)is formed in a manner exposed on the base surface 11B of the n⁻-typeepitaxial layer 10. Accordingly, the p⁺-type channel contact region 34forms a part of the side face of the source trench 33. The p⁺-typechannel contact region 34, in the present preferred embodiment, has itsdeepest portion at a position higher than that of a bottom portion ofthe source trench 33, but the deepest portion is not particularlynecessary at this position. As long as an uppermost portion of thep⁺-type channel contact region 34 (in the present preferred embodiment,the part exposed on the base surface 11B of the n⁻-type epitaxial layer10) is at a position higher than that of the bottom portion of thesource trench 33 and is contactable, said deepest portion may be at thesame depth position as that of the bottom portion of the source trench33 or may be deeper.

In the annular portion 37, an n⁺-type source region 32 and a p-typechannel region 35 are formed in order from the base surface 11B side.Accordingly, the n⁺-type source region 32 and the p-type channel region35 form parts of the side face of the gate trench 8, respectively. Then⁺-type source region 32 is, in the present preferred embodiment, formedwith the same depth as that of the n⁺-type region 21 (refer to FIG. 3 toFIG. 6) and the p⁺-type channel contact region 34.

Also, in the n⁻-type epitaxial layer 10, a p-type layer 38 (for example,having a concentration of 1×10¹⁶ to 1×10¹⁹ cm⁻³) is formed in a mannercontinuing from the p-type channel region 35 and the p⁺-type channelcontact region 34 and the p-type layer 22 (refer to FIG. 4 to FIG. 6)described above. The p-type layer 38 is formed in a manner extendingacross the prismatic portion 36 and the annular portion 37 via thebottom portion of the source trench 33, and its inner region is incontact with the source trench 33 (exposed into the source trench 33).The p-type layer 38 is connected to the p-type channel region 35 at aportion lateral to the source trench 33 of the annular portion 37, andis connected to the p⁺-type channel contact region 34 at a portionlateral to the source trench 33 of the prismatic portion 36. Thus, thep-type channel region 35 and the p⁺-type channel contact region 34 areelectrically connected via the p-type layer 38.

Also, the p-type layer 38 is also formed in a manner extending acrossouter peripheral edges of the outer trench 14 via a bottom portion ofthe outer trench 14, and is connected, at the outer peripheral edges, tothe p-type well 19 extending to the outer peripheral portion 3. Also,the p-type layer 38 may be, as shown in FIG. 2 and FIG. 4, in the innertrench 13, formed only in crossing portions of the lines that constitutethe inner trench 13. In addition, the crossing portions of the innertrench 13 correspond to corner portions of each unit cell 9, and nochannel is formed there at ON-time, or if formed, a small current flowsthrough the channel. Thus, even if the p-type layer 38 is formed in saidcrossing portions in a manner connected to the p-type channel region 35,there is virtually no influence on the device performance.

Also, the p-type layer 38 is, similar to the p-type layer 22, at bottomportions of the gate trench 8 and the source trench 33, formed to bethicker than a part at a side portion of the source trench 33. However,in the prismatic portion 36, a portion lateral to the source trench 33is surrounded by the source trench 33, and ion implantation is uniformlyperformed from its periphery. Therefore, the p-type layer 38 is formedthicker than the part at the bottom portion of the source trench 33, soas to fill a part under the p⁺-type channel contact region 34.

Also, the p-type layer 38 is, in the present preferred embodiment, in apart other than the crossing portions of the inner trench 13 and theouter trench 14, formed across the entire periphery of the annularportion 37 surrounded by the gate trench 8, in a manner not contactingthe gate trench 8 (spaced apart from the gate trench 8). Accordingly, ann⁻-type drain region 20 is disposed at a part of the side face of thegate trench 8 in each unit cell 9, so that a current path at the time ofchannel formation can be secured.

The gate trench 8 is, in the present preferred embodiment, formed in asubstantially U-shape in a sectional view having a side face and abottom face. On an inner surface (side face and bottom face) of the gatetrench 8, a gate insulating film 17 is formed such that its one surfaceand the other surface extend along the inner surface of the gate trench8.

The gate insulating film 17 is, at the bottom portion of the gate trench8, formed to be thicker than a part at a side portion of the gate trench8. In the gate trench 8 having a substantially U-shape in a sectionalview as in the present preferred embodiment, the relatively thick partof the gate insulating film 17 is a part that contacts the bottom faceof the gate trench 8, and the relatively thin part is a part thatcontacts the side face of the gate trench 8. By making the insulatingfilm thick at the bottom portion of the gate trench 8 where electricfield concentration is likely to occur, withstand voltage in the bottomportion of the gate trench 8 can be improved. In addition, the side faceand bottom face sometimes cannot be clearly distinguished depending onthe shape of the gate trench 8, but in that case, it suffices that thegate insulating film 17 that contacts a face in a direction crossing thedepth direction of the gate trench 8 is relatively thick.

Moreover, the inside of the gate insulating film 17 is filled back witha gate electrode 16. In the present preferred embodiment, the gateelectrode 16 is buried in the gate trench 8 such that its upper facebecomes substantially flush with the base surface 11B of the n⁻-typeepitaxial layer 10. The gate electrode 16 is opposed to the p-typechannel region 35 via the gate insulating film 17. In each unit cell 9,by controlling a voltage to be applied to the gate electrode 16, anannular channel along the periphery of the unit cell 9 is formed in thep-type channel region 35. Then, a drain current that flows along theside face of the gate trench 8 toward the base surface 11B of then⁻-type epitaxial layer 10 can be caused to flow to the n⁺-type sourceregion 32 via the channel. A transistor operation of the semiconductordevice 1 is thereby performed.

Similarly, the source trench 33 is also, in the present preferredembodiment, formed in a substantially U-shape in a sectional view havinga side face and a bottom face. On an inner surface (side face and bottomface) of the source trench 33, a source trench insulating film 39 isformed such that its one surface and the other surface extend along theinner surface of the source trench 33.

The source trench insulating film 39 is, at the bottom portion of thesource trench 33, formed to be thicker than a part at a side portion ofthe source trench 33. In addition, the side face and bottom facesometimes cannot be clearly distinguished depending on the shape of thesource trench 33, but in that case, it suffices that the source trenchinsulating film 39 that contacts a face in a direction crossing thedepth direction of the source trench 33 is relatively thick. Moreover,the inside of the source trench insulating film 39 is filled back with atrench buried layer 40. In the present preferred embodiment, the trenchburied layer 40 is buried in the source trench 33 such that its upperface becomes substantially flush with the base surface 11B of then⁻-type epitaxial layer 10.

In the present preferred embodiment, the gate insulating film 17 and thesource trench insulating film 39 are constituted of the same material,and the gate electrode 16 and the trench buried layer 40 are constitutedof the same material.

For example, as the material for the gate insulating film 17 and thesource trench insulating film 39, a film of any of SiO₂, AlON, Al₂O₃,SiO₂/AlON, SiO₂/AlON/SiO₂, SiO₂/SiN, and SiO₂/SiN/SiO₂ can be used, andmore preferably, a film having a SiO₂ film containing nitrogen (N) isused. In addition, SiO₂/AlON means a laminated film of SiO₂ (lower side)and AlON (upper side). Providing a gate insulating film 17 constitutedof a high-dielectric-constant (high-k) film of AlON, Al₂O₃, or the likeallows an improvement in gate withstand voltage, so that devicereliability can be improved. Further, providing a gate insulating film17 constituted of a material having a SiO₂ film containing nitrogen (N)also allows an improvement in channel mobility.

As the material for the gate electrode 16 and the trench buried layer40, polysilicon can be used, and more preferably, n⁺-type polysilicon isused. The n⁺-type polysilicon has a relatively low sheet resistance,which therefore allows increasing transistor switching speed.

In addition, the gate insulating film 17 and the source trenchinsulating film 39 may be constituted of materials different from eachother. The gate electrode 16 and the trench buried layer 40 may also besimilarly constituted of materials different from each other.

The contact holes 29 formed in the surface insulating film 26selectively expose the source trench 33 and the n⁺-type source region 32over the entire surface of the n⁻-type epitaxial layer 10. In thepresent preferred embodiment, a source portion 41 is defined in eachunit cell 9 by the contact hole 29.

Next, a method for manufacturing the semiconductor device 1 described inFIG. 1 to FIG. 7 will be described.

For manufacturing the semiconductor device 1, an n-type impurity isdoped into the surface of a SiC substrate (not shown) while SiC crystalsare caused to grow thereon by epitaxy such as a CVD method, an LPEmethod, or an MBE method. An n⁻-type epitaxial layer 10 is therebyformed on the SiC substrate. A growth surface of the n⁻-type epitaxiallayer 10 formed this time corresponds to the base surface 11B. Inaddition, as the n-type impurity, for example, N (nitride), P(phosphorous), As (arsenic), or the like can be used.

Next, a p-type impurity is selectively ion-implanted from the basesurface 11B of the n⁻-type epitaxial layer 10. A p-type well 19 (p-typechannel region 35) is thereby formed. In addition, as the p-typeimpurity, for example, Al (aluminum), B (boron), or the like can beused. Also, simultaneously with formation of the p-type well 19, therest of the n⁻-type epitaxial layer 10 is formed as an n⁻-type drainregion 20.

Next, an n-type impurity is selectively ion-implanted from the basesurface 11B of the n⁻-type epitaxial layer 10. An n⁺-type region 21 andan n⁺-type source region 32 are thereby simultaneously formed.

Next, the n⁻-type epitaxial layer 10 is selectively etched by use of amask having openings in regions where the gate trench 8, the sourcetrenches 33, and the low step portion 12 are to be formed. The n⁻-typeepitaxial layer 10 is thereby selectively dry-etched so that a gatetrench 8, source trenches 33, and a low step portion 12 are formed, andsimultaneously, a lower surface 11L is formed. In conjunction therewith,the n⁻-type epitaxial layer 10 is defined into a plurality of unit cells9 by the gate trench 8. The unit cells 9 are to have prismatic portions36 and annular portions 37. As an etching gas, for example, a mixed gas(SF₆/O₂ gas) containing SF₆ (sulfur hexafluoride) and O₂ (oxygen), amixed gas (SF₆/O₂/HBr gas) containing SF₆, O₂, and HBr (hydrogenbromide), or the like can be used.

In addition, when locating the lower surface 11L of the outer peripheralportion 3 at a position deeper than the depth of the gate trench 8, itsuffices to further selectively etch the low step portion 12 after theaforementioned etching.

Next, a p-type impurity is selectively ion-implanted from thesemiconductor surface 11 of the n⁻-type epitaxial layer 10. The p-typeimpurity is implanted, for example, in a direction perpendicular tosemiconductor surface 11 of the n⁻-type epitaxial layer 10. A p-typelayer 22, a p-type layer 23, a p-type layer 38, and p-type guard rings25 are thereby simultaneously formed. In addition, these layers 22, 23,38, and 25 may be formed by separate ion implantation steps.

Next, a p-type impurity is selectively ion-implanted from thesemiconductor surface 11 of the n⁻-type epitaxial layer 10. P⁺-typechannel contact regions 34 and a p⁺-type well contact region 24 arethereby simultaneously formed.

Next, the n⁻-type epitaxial layer 10 is thermally treated at 1400° C. to2000° C., for example. The ions of the p-type impurity and n-typeimpurity implanted into the n⁻-type epitaxial layer 10 are therebyactivated.

Next, a gate insulating film 17 and a source trench insulating film 39are simultaneously formed by, for example, thermal oxidization. Inaddition, when the gate insulating film 17 and the source trenchinsulating film 39 are constituted of high-dielectric-constant (high-k)films, it suffices to deposit a film material by a CVD method.

Next, a polysilicon material doped with an n-type impurity is depositedfrom above the n⁻-type epitaxial layer 10 by, for example, a CVD method.The deposition of the polysilicon material is continued until at leastthe gate trench 8 and the source trenches 33 have been completely filledback. Thereafter, by the deposited polysilicon material being patterned,the polysilicon material out of the gate trench 8 (inner trench 13 andouter trench 14) and out of the source trenches 33 is removed in thecell portion 2, and in the outer peripheral portion 3, the polysiliconmaterial remains as an overlapping portion 18. At this time, thepolysilicon material buried in the low step portion 12 is completelyremoved. A gate electrode 16 and a trench buried layer 40 are therebysimultaneously formed.

Next, an insulating material such as SiO₂ is deposited from above then⁻-type epitaxial layer 10 by, for example, a CVD method. A surfaceinsulating film 26 is thereby formed.

Next, a part on the cell portion 2 of the surface insulating film 26 isselectively etched. Only said part is thereby thinned, so that an innerpart 27 and an outer part 28 of the surface insulating film 26 areformed.

Next, by the surface insulating film 26 being selectively etched,contact holes 29 to 31 are simultaneously formed.

Next, a metal material is deposited from above the n⁻-type epitaxiallayer 10 by, for example, a sputtering method. Then, by patterning saidmaterial, a source pad 4, a gate pad 5, and gate fingers 6 aresimultaneously formed. The semiconductor device 1 shown in FIG. 1 toFIG. 7 is obtained through the above steps.

As above, according to the present semiconductor device 1, thesemiconductor surface 11 in which the p-type layer 23 and the p-typeguard rings 25 are formed serves as the lower surface 11L at a depthposition equivalent to or deeper than the depth of the gate trench 8.The thickness of the n⁻-type epitaxial layer 10 from the bottom portionof the gate trench 8 to a back surface of the n⁻-type epitaxial layer 10can thereby be made thicker than the thickness from the p-type layer 23and the p-type guard rings 25 to said back surface. As a result, anelectric field imposed on a section between the surface side and backsurface side of the n⁻-type epitaxial layer 10 can be made to be stablyshared by the p-type layer 23 and the p-type guard rings 25 of the outerperipheral portion 3. Because a stable electric field distribution canaccordingly be formed in the n⁻-type epitaxial layer 10 withoutdepending on the depth of the gate trench 8, an electric fieldconcentration to the bottom portion of the gate trench 8 can besatisfactorily relaxed.

Also, as shown in FIG. 2, the gate trench 8 is formed in a region underthe gate fingers 6, but the line-shaped contact trenches 15 are onlyformed in a manner running across the gate fingers 6, and moreover, theterminal portions of the contact trenches 15 are disposed furtheroutside than the gate fingers 6. In other words, because trench cornerportions (for example, corners etc., in the crossing portions of theinner trench 13) where an electric field is likely to concentrate when agate voltage is applied are not disposed under the gate fingers 6, thereliability and resistance of the gate insulating film 17 can beimproved.

Also, as shown in FIG. 7, the p-type layer 38 is also formed in a mannerextending across the outer peripheral edges of the outer trench 14 viathe bottom portion of the outer trench 14. A depletion layer can therebybe generated from a junction (p-n junction) between the p-type layer 38and the n⁻-type epitaxial layer 10 (n⁻-type drain region 20). Moreover,because the depletion layer keeps equipotential surfaces away from theouter trench 14, electric fields to be imposed on the bottom portion ofthe outer trench 14 can be relaxed. Breakdown in the bottom portion ofthe outer trench 14 can accordingly be prevented.

Also, as shown in FIG. 5 and FIG. 6, in a region under the gate fingers6, the n⁺-type region 21 is formed in the semiconductor surface 11 (basesurface 11B) of the n⁻-type epitaxial layer 10. Because an n⁺-typesemiconductor region has a higher oxidation rate than that of an n⁻-typesemiconductor region, in a region under the gate fingers 6, a thickoxide film can be selectively formed in an upper portion of the gatetrench 8 (contact trench 15) when the gate insulating film 17 is formedby thermal oxidization. An electric field to be imposed on an upper edgeof the contact trench 15 when a gate voltage is applied can thereby beweakened to prevent a dielectric breakdown of the gate insulating film17.

Also, as shown in FIG. 3 and FIG. 4, by selectively thinning the surfaceinsulating film 26 (inner part 27) of the cell portion 2, a differencein level (unevenness) between the semiconductor surface 11 (devicesurface) in the source portions 41 within the contact holes 29 and asurface of the surface insulating film 26 can be reduced. When a sourcepad 4 is buried in the contact holes 29 to form a source pad 4 on thesurface insulating film 26, the flatness of the source pad 4 can therebybe further improved.

On the other hand, the thickness of the surface insulating film 26(outer part 28) of the outer peripheral portion 3 can be designedseparately from the thickness of the inner part 27. Thus, designing withsuch a thickness so as not to influence the electric field distributionin the outer peripheral portion 3 allows maintaining breakdowncharacteristics. In other words, according to this arrangement, at thetime of an improvement in flatness of the source pad 4, variation inbreakdown characteristics and a dielectric breakdown failure due to thevariation can be prevented.

Also, as shown in FIG. 7, the trench buried layer 40 is buried in thesource trenches 33 via the trench insulating film 39. Therefore, on thesurface of the n⁻-type epitaxial layer 10 (device surface), a differencein level (unevenness) between the source portions 41 exposed from thecontact holes 29 and other parts can be reduced. The flatness of thesource pad 4 on said device surface can thereby be improved. Thus, when,for example, a wire is bonded to the surface of the source pad 4,adhesion between the source pad 4 and the wire can be improved. As aresult, the wire can be satisfactorily bonded, so that the wire bondingportion can be improved in reliability. Further, because the source pad4 is excellent in flatness, destruction of the device by ultrasonicvibration and pressure can be prevented at the time of wire bonding, anda decline in assembling yield can be prevented.

On the other hand, a concentration of equipotential surfaces in avicinity of the bottom portion of the gate trench 8 can be prevented bythe source trench 33, so that a potential gradient in the vicinity ofthe bottom portion can be made gradual. Therefore, an electric fieldconcentration to the bottom portion of the gate trench 8 can be relaxed.Further, the p⁺-type channel contact region 34 is formed in the topportion of the prismatic portion 36 and is disposed at a position higherthan that of the bottom portion of the source trench 33. Thus, even whenthere is formed a source trench 33, contact with the p-type channelregion 35 can be reliably made via the p⁺-type channel contact region34. In other words, at the time of an improvement in flatness of thesource pad 4, a degradation in device performance such as gate withstandvoltage and contact performance with the p-type channel region 35 can beprevented.

Further, in the present preferred embodiment, because the p-type layer38 is formed around the source trench 33, a depletion layer can begenerated from a junction (p-n junction) between the p-type layer 38 andthe n⁻-type drain region 20. Moreover, because the depletion layer keepsequipotential surfaces away from the gate trench 8, electric fields tobe imposed on the bottom portion of the gate trench 8 can be furtherrelaxed.

Also, in the present preferred embodiment, because a SiC device in whichlatch-up is unlikely to occur as compared with a Si device is used, thep⁺-type channel contact region 34 and the p-type channel region 35 canbe provided at positions separated from each other by the source trench33. That is, in a Si device, because latch-up is relatively likely tooccur, it is preferable to dispose the p⁺-type channel contact region 34near the p-type channel region 35 to reduce the distance between theregions 34 and 35 as short as possible so as to lower a base resistancebetween said regions 34 and 35. On the other hand, in such a SiC deviceas the present semiconductor device 1, because latch-up is relativelyunlikely to occur and the importance of considering a base resistancebetween the regions 34 and 35 is low, the p⁺-type channel contact region34 does not need to be disposed near the p-type channel region 35. Thus,the p⁺-type channel contact region 34 and the p-type channel region 35can be provided at positions separated from each other by the sourcetrench 33 to electrically connect the regions 34 and 35 by a routethrough the bottom portion of the source trench 33.

Also, because the source trench insulating film 39 is disposed outsideof the trench buried layer 40, flow of an off-leakage current betweenthe n⁻-type epitaxial layer 10 and the source pad 4 can be prevented.Specifically, the p-type layer 38 is, at a side portion of the sourcetrench 33, thinner than a part at the bottom portion of the sourcetrench 33 because ions are unlikely to enter a portion lateral to thesource trench 33 at the time of ion implantation. Therefore, when a highvoltage is applied at OFF-time, an off-leakage current may flow passingthrough the thin part of the p-type layer 38. Therefore, forming asource trench insulating film 39 allows reliably interrupting a leakagecurrent by the source trench insulating film 39 even if an off-leakagecurrent passes through the p-type layer 38.

Also, if the trench buried layer 40 buried in the source trench 33 ispolysilicon, when forming contact holes 29 in the surface insulatingfilm 26 made of SiO₂, the trench buried layer 40 (polysilicon layer) canbe used as an etching stopper. Therefore, control of the step of saidcontact etching can be simplified.

Also, because the source trenches 33 are formed simultaneously with thegate trench 8, the source trenches 33 can be simply formed free frommisalignment without increasing the manufacturing process. Further, ifthe source trenches 33 and the gate trench 8 are the same width, theetching rate for the source trenches 33 can be made the same as that forthe gate trench 8, so that etching for forming the source trenches 33can be stably controlled.

Next, modifications of the cell portion 2 will be described withreference to FIG. 8 to FIG. 12.

FIG. 8 to FIG. 12 are views showing first to fifth modifications of thecell portion 2. In FIG. 8 to FIG. 12, parts corresponding to therespective portions shown in FIG. 7 described above are shown with thesame reference signs.

In the form of FIG. 7, the trench buried portion buried in the sourcetrench 33 consists of the source trench insulating film 39 and thetrench buried layer 40 (polysilicon layer), but as shown in FIG. 8, itmay consist only of an insulating layer 42 that fills back the sourcetrenches 33.

As the material for the insulating layer 42, SiO₂ can be used, and morepreferably, SiO₂ containing phosphorus (P) or boron (B) is used. As suchSiO₂, for example, PSG (phosphorus silicate glass) or PBSG (phosphorusboron silicate glass) can be used.

A process for manufacturing the semiconductor device of the form shownin FIG. 8 is substantially the same as the steps described in theforegoing. However, after a gate electrode 16 and a trench buried layer40 are formed, the trench buried layer 40 is selectively etched to beremoved, so that the source trenches 33 are made hollow. Then, a surfaceinsulating film 26 is formed on the n⁻-type epitaxial layer 10 tothereby fill back the source trenches 33 by use of a part of the surfaceinsulating film 26. The source trench insulating film 39 and the surfaceinsulating film 26 are thereby integrated inside the source trenches 33,so that an insulating layer 42 is formed.

According to this arrangement, because the source trenches 33 are filledwith the insulating layer 42, flow of an off-leakage current between then⁻-type epitaxial layer 10 and the source pad 4 can be effectivelyprevented.

Also, if the insulating layer 42 is SiO₂ containing phosphorous orboron, because the melting point of SiO₂ falls, the process for buryingthe insulating layer 42 can be simplified.

Also, as shown in FIG. 9, the trench filling portion buried in thesource trench 33 may consist only of a polysilicon layer 43 that fillsback the source trenches 33. As the material for the polysilicon layer43, p⁺-type polysilicon is preferably used.

A process for manufacturing the semiconductor device of the form shownin FIG. 9 is substantially the same as the steps described in theforegoing. However, after a gate insulating film 17 and a source trenchinsulating film 39 are formed, the source trench insulating film 39 isselectively etched to be removed, so that the source trenches 33 aremade hollow. Then, by polysilicon being deposited from above the n⁻-typeepitaxial layer 10, the source trenches 33 are filled back with thatpolysilicon. A gate electrode 16 and a polysilicon layer 43 are therebysimultaneously formed.

According to this arrangement, because the polysilicon layer 43 isburied in the source trenches 33, when forming contact holes 29 in thesurface insulating film 26 made of SiO₂, the polysilicon layer 43 can beused as an etching stopper. Therefore, control of the step of saidcontact etching can be simplified.

Also, if the polysilicon layer 43 is p⁺-type polysilicon, the p⁺-typechannel contact region 34 and the p-type channel region 35 can beelectrically connected by use of the polysilicon layer 43. Because thelength of a current path between the regions 34 and 35 can thereby bereduced, a base resistance therebetween can be reduced. As a result,latch-up can be satisfactorily prevented. Further, because the p⁺-typechannel contact region 34 is in contact with the polysilicon layer 43 ata side face of the source trench 33, a contact resistance therebetweencan also be reduced. The reduction in contact resistance alsocontributes to a reduction in the base resistance between the regions 34and 35.

Also, in the form of FIG. 7, the source trench 33 is formed in anannular shape in a region surrounded by the annular n⁺-type sourceregion 32, but as shown in FIG. 10, a source trench 44 in a recessedshape showing a quadrangular shape in a plan view may be formed in aregion surrounded by the n⁺-type source region 32. In this case, ap⁺-type channel contact region 45 may be formed in a surface portion ofthe p-type layer 38 at a bottom portion of the source trench 44.

Also, in the form of FIG. 7, the gate electrode 16 is a layer made onlyof polysilicon that fills back the inside of the gate insulating film17, but as shown in FIG. 11, it may consist of a base film 46 made ofpolysilicon formed, on the gate insulating film 17, such that its onesurface and the other surface extend along the inner surface of the gatetrench 8 and a buried metal 47 containing at least one of Mo, W, Al, Pt,Ni, and Ti buried inside of the base film 46. In this case, the trenchburied portion within the source trench 33 may also similarly consist ofa base film 48 made of polysilicon formed, on the source trenchinsulating film 39, such that its one surface and the other surfaceextend along the inner surface of the source trench 33 and a buriedmetal 49 made of the same material as that for the buried metal 47buried inside of the base film 48.

According to this arrangement, a metal gate using the buried metal 47can make gate resistance relatively low as compared with that of apolysilicon gate, which therefore allows increasing transistor switchingspeed.

Also, a trench-gate type structured MISFET is formed in the cell portion2 of FIG. 7, but as shown in FIG. 12, a planar type structured MISFETmay be formed in the cell portion 2.

That is, in the form shown in FIG. 12, p-type wells 19 are arrayed in amatrix shape (in an array) in the cell portion 2 in a mannercorresponding to the respective unit cells 9. In a surface portion ofeach p-type well 19, an annular n⁺-type source region 50 is formed in amanner exposed on the base surface 11B. Moreover, a gate electrode 51 isdisposed via a gate insulating film 52 in a manner opposed to a regionbetween an outer peripheral edge of each p-type well 19 and an outerperipheral edge of the n⁺-type source region 50. The surface insulatingfilm 26 covers the gate electrode 51.

Second Preferred Embodiment

FIG. 13 and FIG. 14 are schematic sectional views of a semiconductordevice according to a second preferred embodiment of the presentinvention, and show sectional structures corresponding to FIG. 3 andFIG. 4, respectively. In FIG. 13 and FIG. 14, parts corresponding to therespective portions shown in FIG. 3 and FIG. 4 described above are shownwith the same reference signs.

In the first preferred embodiment described above, a boundary betweenthe base surface 11B and the lower surface 11L due to the low stepportion 12 is set further inside than a contact position of the sourcepad 4 with respect to the p-type well 19 extending across the cellportion 2 and the outer peripheral portion 3, but as shown in FIG. 13and FIG. 14, it may be set outside. In this case, the p⁺-type wellcontact region 24 is formed at a position spaced apart to the outsidewith respect to the n⁺-type region 21 in an inner region of the p-typewell 19.

According to this arrangement, both the p⁺-type channel contact regions34 and the p⁺-type well contact region 24 can be formed in the basesurface 11B, which can therefore make mask alignment easy at the time ofion injection when these regions 24 and 34 are formed. Of course, thesame effects as those of the first preferred embodiment can also berealized.

Third Preferred Embodiment

FIG. 15 and FIG. 16 are schematic sectional views of a semiconductordevice according to a third preferred embodiment of the presentinvention, and show sectional structures corresponding to FIG. 3 andFIG. 4, respectively. In FIG. 15 and FIG. 16, parts corresponding to therespective portions shown in FIG. 3 and FIG. 4 described above are shownwith the same reference signs.

In the first preferred embodiment described above, the low step portion12 is formed in the outer peripheral portion 3, but in the present thirdpreferred embodiment, no low step portion 12 is formed in the outerperipheral portion 3, and the outer peripheral portion 3 has asemiconductor surface 11 at the same height position as that of the basesurface 11B of the cell portion 2.

According to this arrangement, both the p⁺-type channel contact regions34 and the p⁺-type well contact region 24 can be formed in the basesurface 11B, which can therefore make mask alignment easy at the time ofion injection when these regions 24 and 34 are formed. Of course, thesame effects as those of the first preferred embodiment can also berealized.

First Reference Embodiment

FIG. 17 and FIG. 18 are schematic sectional views of a semiconductordevice according to a first reference embodiment of the presentinvention, and show sectional structures corresponding to FIG. 3 andFIG. 4, respectively. In FIG. 17 and FIG. 18, parts corresponding to therespective portions shown in FIG. 3 and FIG. 4 described above are shownwith the same reference signs.

In the first preferred embodiment described above, the surfaceinsulating film 26 is formed such that the inner part 27 on the cellportion 2 becomes thinner than the outer part 28 on the outer peripheralportion 3, but as shown in FIG. 17 and FIG. 18, it may be formed suchthat the inner part 27 on the cell portion 2 has the same thickness asthat of the outer part 28 on the outer peripheral portion 3. The filmthickness may be, for example, about 5500 Å to 20000 Å.

Second Reference Embodiment

FIG. 19 and FIG. 20 are schematic sectional views of a semiconductordevice according to a second reference embodiment of the presentinvention, and show sectional structures corresponding to FIG. 3 andFIG. 4, respectively. In FIG. 19 and FIG. 20, parts corresponding to therespective portions shown in FIG. 3 and FIG. 4 described above are shownwith the same reference signs.

The structure in FIG. 19 and FIG. 20 is an example for which thearrangement of the surface insulating film 26 having a uniform thicknessof the first reference embodiment described above is combined with thestructure of the second preferred embodiment.

Fourth Preferred Embodiment

FIG. 21 is a schematic sectional view of a semiconductor deviceaccording to a fourth preferred embodiment of the present invention, andshows a sectional structure corresponding to FIG. 3. In FIG. 21, partscorresponding to the respective portions shown in FIG. 3 described aboveare shown with the same reference signs.

In the first preferred embodiment described above, the voltage resistantstructure of the outer peripheral portion 3 consists only of p-typesemiconductor regions, like the p-type layer 23 and the p-type guardrings 25, but as shown in FIG. 21, it may be arranged to include atrench formed in the lower surface 11L and a p-type semiconductor regionformed at a bottom portion of the trench. In this case, the trench mayhave a conductive material buried therein via an insulating film. In thepresent preferred embodiment, guard rings 55 are formed each including atrench 53 formed in the lower surface 11L and having an annular shape tosurround the cell portion 2 and a p-type layer 54 which is formed atbottom and side portions of the trench 53 and an inner region of whichis in contact with the trench 53. In the trenches 53, a polysiliconlayer 57 is buried via a trench insulating film 56.

By this arrangement as well, the same effects as those of the firstpreferred embodiment can be realized.

Although preferred embodiments of the present invention have beendescribed above, the present invention can be embodied in other forms.

For example, an arrangement may be adopted in which the conductivitytype of each semiconductor part of the semiconductor device 1 isinverted. For example, in the semiconductor device 1, the p-type partsmay be n-type and the n-type parts may be p-type.

Also, in the semiconductor device 1, the layer that constitutes asemiconductor layer is not limited to an n⁻-type epitaxial layer made ofSiC, and may be a layer or the like made of GaN, diamond, or Si.

Also, each unit cell 9 is not limited to a square shape in a plan view(quadrangular shape), but may have another shape such as, for example, atriangular shape in a plan view, a pentagonal shape in a plan view, or ahexagonal shape in a plan view.

The semiconductor device of the present invention can be incorporatedin, for example, a power module for use in an inverter circuit thatconstitutes a drive circuit for driving an electric motor available as apower source for an electric vehicle (including a hybrid vehicle), anelectric train, an industrial robot, and the like. Additionally, thesemiconductor device of the present invention can also be incorporatedin a power module for use in an inverter circuit that converts electricpower generated by a solar cell, a wind power generator, and other powergenerators (particularly, private electric generators) so as to bematched with electric power from commercial power sources.

Also, the features grasped from the disclosures of the preferredembodiments described above may be combined with each other even amongdifferent preferred embodiments. Also, the components presented in therespective preferred embodiments may be combined within the scope of thepresent invention.

The preferred embodiments of the present invention are merely specificexamples used to clarify the technical content of the present invention,and the present invention should not be interpreted as being limited tothese specific examples, and the spirit and scope of the presentinvention shall be limited solely by the accompanying claims.

The present application corresponds to Japanese Patent Application No.2013-43407 filed on Mar. 5, 2013 in the Japan Patent Office, and theentire disclosure of this application is incorporated herein byreference.

REFERENCE SIGNS LIST

-   -   1 Semiconductor device    -   2 Cell portion    -   3 Outer peripheral portion    -   4 Source pad    -   5 Gate pad    -   6 Gate finger    -   7 Removal region    -   8 Gate trench    -   9 Unit cell    -   10 N⁻-type epitaxial layer    -   11 Semiconductor surface    -   11B Base surface    -   11L Lower surface    -   12 Low step portion    -   13 Inner trench    -   14 Outer trench    -   15 Contact trench    -   16 Gate electrode    -   17 Gate insulating film    -   18 Overlapping portion    -   19 P-type well    -   20 N⁻-type drain region    -   21 N⁺-type region    -   22 P-type layer    -   23 P-type layer    -   24 P⁺-type well contact region    -   25 P-type guard ring    -   26 Surface insulating film    -   27 Inner part    -   28 Outer part    -   29 Contact hole    -   30 Contact hole    -   31 Contact hole    -   32 N⁺-type source region    -   33 Source trench    -   34 P⁺-type channel contact region    -   35 P-type channel region    -   36 Prismatic portion    -   37 Annular portion    -   38 P-type layer    -   39 Source trench insulating film    -   40 Trench buried layer    -   41 Source portion    -   42 Insulating layer    -   43 Polysilicon layer    -   44 Source trench    -   45 P⁺-type channel contact region    -   46 Base film    -   47 Buried metal    -   48 Base film    -   49 Buried metal    -   50 N⁺-type source region    -   51 Gate electrode    -   52 Gate insulating film    -   53 Trench    -   54 P-type layer    -   55 Guard ring    -   56 Trench insulating film    -   57 Polysilicon layer

The invention claimed is:
 1. A wide band gap semiconductor devicecomprising: a semiconductor substrate; a first gate insulating film onthe semiconductor substrate; a first gate electrode on the first gateinsulating film; a second gate insulating film on the semiconductorsubstrate; and a second gate electrode on the second gate insulatingfilm; and an electrode having a source potential formed over thesemiconductor substrate, the electrode being outwardly away from thefirst gate electrode and the second gate electrode, wherein a portion ofa surface of the semiconductor substrate between a center of the firstgate electrode and a center of the second gate electrode is recessedcompared to a portion under the center of the first gate electrode andthe center of the second gate electrode, wherein the semiconductorsubstrate comprises an outermost recessed portion that includes arecess, and the electrode is arranged between the first and second gateelectrodes and the recess of the outermost recessed portion and does notintrude into the recess of the outermost recessed portion.
 2. The wideband gap semiconductor device according to claim 1, wherein an impurityregion of P-type material is formed at a surface of the outermostrecessed portion.
 3. The wide band gap semiconductor device according toclaim 2, wherein a source electrode is formed over the semiconductorsubstrate, the electrode electrically connected to the source electrode.4. The wide band gap semiconductor device according to claim 3, whereina source contact region is formed in a region between the first gateelectrode and the second gate electrode.
 5. The wide band gapsemiconductor device according to claim 4, wherein at least one of thefirst gate electrode and the second gate electrode is a planer type gateelectrode.
 6. The wide band gap semiconductor device according to claim5, wherein a gate finger is formed between the source electrode and theoutermost recessed portion of the semiconductor substrate in a crosssection.
 7. The wide band gap semiconductor device according to claim 6,wherein a height of the gate finger is approximately the same as aheight of the source electrode.
 8. The wide band gap semiconductordevice according to claim 7, wherein the source electrode is made of amaterial including an aluminum.
 9. The wide band gap semiconductordevice according to claim 8, wherein the gate finger is made of amaterial including an aluminum.
 10. The wide band gap semiconductordevice according to claim 9, wherein a peripheral source electrode isformed between the gate finger and the outermost recessed portion of thesemiconductor substrate.
 11. The wide band gap semiconductor deviceaccording to claim 10, wherein the semiconductor substrate is made ofSiC, GaN, or diamond.
 12. The wide band gap semiconductor deviceaccording to claim 11, further comprising a titanium material betweenthe source electrode and the semiconductor substrate at the sourcecontact region.
 13. The wide band gap semiconductor device according toclaim 12, wherein the gate insulating film is made of a materialincluding silicon.
 14. The wide band gap semiconductor device accordingto claim 13, wherein the gate electrode is made of a material includinga polysilicon.
 15. The wide band gap semiconductor device according toclaim 14, wherein a surface of the source electrode is dented accordingto the surface of the semiconductor substrate.
 16. The wide band gapsemiconductor device according to claim 14, wherein a depth of therecess of the portion of the surface of the semiconductor substratebetween the center of the first gate electrode and the center of thesecond gate electrode is approximately the same as a depth of the recessof the outermost recessed portion.
 17. The wide band gap semiconductordevice according to claim 14, wherein the recess of the portion ofsurface of the semiconductor substrate between the center of the firstgate electrode and the center of the second gate electrode is fabricatedat a same time with the recess of the outermost recessed portion.